Centrality, Rapidity and pT dependence of isolated prompt photon production in Pb+Pb collisions with the ATLAS detector at the LHC PETER STEINBERG, BROOKHAVEN NATIONAL LABORATORY QUARK MATTER 2014 MAY 19, 2014 Motivation for inclusive photons • Photons are a penetrating probe of the hot dense medium • Do not interact strongly, so are sensitive to the initial state of the nucleus-nucleus collision • Important test of our understanding of initial state • Should scale with nuclear thickness (initial parton flux) • Sensitive to nPDF effects 2 2 R Pb g (x,Q =100 GeV ) 1.4 FGS10 HKN 1.2 nDS 1 0.8 0.6 0.4 0.2 (EPS09, nDS, HKN, FGS) EPS09 Salgado et al, arxiv:1105.3919 0 10-5 10-4 10-3 10-2 x • Potential sensitivity to jet-medium effects • In principle jets could convert to photons in-medium (Gale et al) 2 10-1 1 Scope of measurement • This result covers a wide range of nuclear geometry and wide phase space. • 4 centrality regions (40-80%, 20-40%, 10-20%, 0-10%) • 2 pseudorapidity regions (central |η|<1.37 and forward 1.52<|η|<2.37) • 11 pT regions (logarithmic from 22.1-280 GeV) ! • Also measure ratio of yields between forward and central η interval • Useful to cancel various effects ! • Yields scaled by the nuclear thickness function TAA and compared to JETPHOX 1.3.0 NLO pQCD 3 Simulation data samples • In order to simulate both contributions to the photon cross section — direct and fragmentation — two simulated datasets were run • MC11 PYTHIA 6.4 direct photon (17-280 GeV) • MC11 PYTHIA 6.4 dijets (17-560 GeV) with hard photon filter • O(3 billion) events needed to get 3 million photons ! • Simulated pp events are merged with real minimum bias data events, and reconstructed as if they were real data • “Data overlay” - perfect description of underlying event. 4 Data selection • Events from 2011 Pb+Pb dataset, • Integrated luminosity 0.14 nb (1/Nevt) dNevt/dΣET [1/TeV] trigged on a 16 GeV electromagnetic deposition within 0.2x0.1 area in ∆ηx∆ɸ -1 ! • Minimum-bias event selections used to clean events 102 ATLAS Preliminary Pb+Pb sNN=2.76 TeV Minimum bias 16 GeV EM trigger 40 GeV tight photons γ -1 -1 LMB int =5 µ b , Lint=0.14 nb 1 10-2 10-4 40-80% 20-40% 10-20% 0-10% 10-6 • ZDC coincidence, ∆tMBTS < 5 ns, reconstructed vertex 10-8 0 ! 1 2 3 FCal ΣET [TeV] • Centrality selection performed using ATLAS forward calorimeter (3.2<|η| <4.9) 5 Photon reconstruction & shower shape variables • Underlying event removed in Cells in Layer 3 ∆ϕ×∆η = 0.0245×0.05 Trigge • Trigge Tow r ∆ϕ = 0er .0982 47 0m 1 Square cells in Layer 2 ∆ϕ = 0 .0245 ∆η = 0 .025 m/8 = 4.69 m m ∆η = 0 .0031 Strip cells in Layer 1 37.5m η π0 Figure 5.4: Sketch of a barrel module where the different layers are clearly visible with the ganging of electrodes in f . The granularity in h and f of the cells of each of the three layers and of the trigger towers is also shown. • First layer variables: • 2 m 00 m 1.7X0 ∆ϕ=0. 0245x 36.8m 4 mx =147.3 4 mm ϕ γ Fraction of cluster ET in hadronic section 16X0 4.3X0 Containment and width of showers • Hadronic leakage variables: r Towe ∆η = 0 r .1 m η=0 • Second layer variables: • 3 2X0 15 ∆η=0.1 regions from HI jet reconstruction • Photon clusters seeded using sliding window in 2nd calorimeter layer. • Photon identification is based on 9 shower shape variables Hadronic 5.2.2 Rejects candidates with two showers 6 Barrel geometry The barrel electromagnetic calorimeter [107] is made of two half-barrels, centred around the zaxis. One half-barrel covers the region with z > 0 (0 < h < 1.475) and the other one the region with z < 0 ( 1.475 < h < 0). The length of each half-barrel is 3.2 m, their inner and outer diameters are 2.8 m and 4 m respectively, and each half-barrel weighs 57 tonnes. As mentioned above, the barrel calorimeter is complemented with a liquid-argon presampler detector, placed in front of its inner surface, over the full h-range. ATLAS Preliminary Pb+Pb sNN=2.76 TeV 0-10% Central p =35-44 GeV T |η|<1.37 Data Simulation ATLAS Preliminary Pb+Pb sNN=2.76 TeV 0-10% Central p =35-44 GeV T |η|<1.37 Data Simulation 103 102 102 Entries [/0.006] 103 Entries [/0.00025] Entries [/0.02] Selecting photons with shower shape variables 103 10 10 1 1 1 0.006 0.008 0.01 0.012 w η,2 103 ATLAS Preliminary Pb+Pb sNN=2.76 TeV 40-80% Central p =35-44 GeV T |η|<1.37 Data Simulation ATLAS Preliminary Pb+Pb sNN=2.76 TeV 40-80% Central p =35-44 GeV T |η|<1.37 Data Simulation 102 10 10 1 1 0.4 0.6 0.8 w s,3 Width in three strips -0.1 Entries [/0.006] 102 0.8 w s,3 Entries [/0.00025] Entries [/0.02] 103 0.6 small shower-shape corrections applied to account for observed data vs. MC differences 102 10 0.4 ATLAS Preliminary Pb+Pb sNN=2.76 TeV 0-10% Central p =35-44 GeV T |η|<1.37 Data Simulation 103 102 0 0.1 Rhad ATLAS Preliminary Pb+Pb sNN=2.76 TeV 40-80% Central p =35-44 GeV T |η|<1.37 Data Simulation 10 1 0.006 0.008 0.01 0.012 w η,2 η width in 2nd layer -0.1 0 0.1 Rhad Hadronic leakage fraction “Tight” selection - satisfies 9 shower shape selections per photon, tuned on photons in HI events “Non-tight” selection - Background enhanced by selecting clusters that fail selections which usually reject having two distinct showers, or two merged showers 7 P(ET,iso) P(ET,iso) Isolation selection 0.04 0.04 0.02 0.02 P(ET,iso) -20 0.08 ET,iso(R=0.3) -20 40 [GeV] 0.08 0.06 0.04 0.04 0.02 0.02 P(ET,iso) ! • Simulated isolation distribution, normalized to data for ET,iso<0, showing increasing fluctuations in central events ! • Non-tight photons from data, normalized for ET,iso>8 GeV, showing how jets fill the tails in the isolation distributions 20 0.06 -20 0.1 0 20 -20 40 ET,iso(R=0.3) [GeV] 0.1 0.05 -20 0.15 0 20 40 ET,iso(R=0.3) [GeV] 10-20% Central 0 20 40 ET,iso(R=0.3) [GeV] 0.05 0 20 -20 40 0.15 ET,iso(R=0.3) [GeV] 0.1 0.1 0.05 0.05 -20 0 20 -20 40 ET,iso(R=0.3) [GeV] 8 ATLAS Preliminary Pb+Pb sNN=2.76 TeV Lint =0.14 nb-1 0-10% Central p =35-44 GeV T |η|<1.37 20-40% Central P(ET,iso) • “Isolated”: ET,iso(R=0.3) < 6 GeV 0 P(ET,iso) depth of the calorimeter in a ring of R=0.3 around the photon direction, with photon removed 0.06 P(ET,iso) • ET,iso is the transverse energy in the full Tight photons0.08 Non-tight photons γ Simulation 0.06 P(ET,iso) 0.08 0 20 40 ET,iso(R=0.3) [GeV] 40-80% Central 0 20 40 ET,iso(R=0.3) [GeV] P(ET,iso) P(ET,iso) Double sideband method: ideal 0.08 D A 0 B 10 0.06 0.04 0.04 0.02 20 30 ET(R =0.3) [GeV] iso -20 0.08 ATLAS Preliminary Pb+Pb sNN=2.76 TeV Lint =0.14 nb-1 0-10% Central p =35-44 GeV T Nontight |η|<1.37 Tight B A 0.02 P(ET,iso) Tight C P(ET,iso) Non-tight Tight photons0.08 Non-tight photons γ Simulation 0.06 20 -20 40 00 E 10(R=0.3) 20 [GeV] 0.08 T,iso D C 0 20 40 0 10 20 E (R=0.3) [GeV] T,iso Basic principle is straightforward: use non-tight 0.04 photons to extrapolate 0.02 non-isolated/non-tight photons 0.02 into the signal (tight, isolated region): 10-20% Central 0.06 P(ET,iso) P(ET,iso) 0.06 of the double side-band approach, showing the two axes for partitioning photon the “signal region” (tight and isolated photons) for which efficiencies are defined, 0.04 t, non-isolated photons, region C contains non-tight isolated photons, and region nd non-isolated photons. 0 20 -20 40 ible compared with other uncertainties. Hence, no reweighting is-20 applied to the E (R=0.3) [GeV] 0.1 ccount for any systematic di↵erence between the distributions in 0.1 data and MC, the T,iso varied in both data and MC as described in Sec. 8. nt, only the highest pT photon is considered, since far less than 1% of events with are expected to have a second hard photon, and the overall rate of additional tight, 0.05 ates in the data was found to be less than 1% relative to that for 0.05 the highest-energy ter application of the tight selection and an isolation criterion of ET,iso < 6 GeV y sample, there are approximately 51,000 candidates with pT > 22 GeV within 9 andidates within 1.52 < |⌘| < 2.37. 0 20 Yield = A - B (C/D) = A - C (B/D) (If A/B = C/D, then yield =0) 40 ET,iso(R=0.3) [GeV] 20-40% Central Non-tight NN -1 int T P(ET,iso) P(ET,iso) iso P(ET,iso) T P(ET,iso) Tight nt, only the highest pT photon is considered, since far less than 1% of events with sigrate of additional tight, are expected to have a second hard photon, and the overall A 0.05 661 ates in the data was found to be less than 1% relative to that for the highest-energy ter application of the tight selection and an isolation criterion of ET,iso < 6 GeV 662 i GeV within y sample, there are approximately 51,000 candidates with pT > 22 10 andidates withinsig 1.52 < |⌘| < 2.37. on P(ET,iso) P(ET,iso) n. 652 C and D), then the double sideband approach utilizes ts 653 the ratio of counts in C to D to extrapolate the measured ue Double sideband method: actual 654 number of counts in region B to correct the measured e655 number of counts in region 0.08 A, i.e. Tight photons0.08 1] ATLAS Preliminary Non-tight photons Pb+Pb s =2.76 TeV γ Simulation ed L =0.14 nb obs C D 0.06 0.06 N 0-10% Central sig obs obs C Tight is N = NA NB (2) p =35-44 GeV obs Nontight |η|<1.37 0.04 0.04 N D ne B 0.02 0.02 A A B e, 656 Leakage of signal into the background regions needs to C D t” 657 removed before attempting to extrapolate into the signal -20 0 20 -20 40 0 20 40 0 10 20 30 0 10 20 0 10 20 0.08 ET,iso(R=0.3) [GeV] 0.08 ET,iso(R=0.3) [GeV] E (R =0.3) [GeV] n- 658 region. A set of “leakage factors”, ci , are calculated to he 10-20% Central 0.06 0.06 of the659 double extrapolate side-band approach,the showing the two axes forsignal partitioning photon in region number of events A into In reality, signal photons can “leak” into the sidebands: the “signal region” (tight and isolated photons) for which efficiencies are defined, ch 660 the other regions. 0.04 0.04 t, non-isolated photons, region C contains non-tight isolated photons, and region use simulations to estimate leakage factors using signal-only: c X = NX/NA ch nd non-isolated photons. 0.02 0.02 ⇣ ⌘ sig obs ns ⇣ ⌘ NC cC NA obs=Data sig sig obs Hence, no obsreweighting is-20applied to the 0 20 -20 40 (3) 0 20 40 ible compared withN other uncertainties. ⇣ ⌘ = N N c N B A B E (R=0.3) sig [GeV] ET,iso(R=0.3) [GeV] A A 0.1 ccount for any systematic di↵erence between the distributions in 0.1 data and MC, obs the T,iso sig=signal N c N D eD A varied in both data and MC as described in Sec. 8. 20-40% Central Solvedfactors for N are with calculated quadratic formula, and The leakage using the 0.05 sig is zero sig when AD=BC) only one root is physical (the one that PYTHIA+data sample as c = Ni /NA , where 0.5 0.5 40-80%, |η|<1.37 102 2×102 2 10 2 2×10 30 40 2 10 2 2×10 1 photon p T [GeV] 1 photon p T [GeV] 1 photon p T [GeV] 0.5 0.5 0.5 0.5 40-80%, 1.52<|η|<2.37 0 20-40%, 1.52<|η|<2.37 0 30 40 102 2×102 photon p T [GeV] 102 1 2×102 photon p T [GeV] 30 40 102 A Nobs 2×102 photon p T [GeV] 0-10%, 1.52<|η|<2.37 10-20%, 1.52<|η|<2.37 0 0 30 4050 A Nsig 0 0 30 40 P = 0-10%, |η|<1.37 10-20%, |η|<1.37 Purity 30 40 1 ATLAS Preliminary Pb+Pb sNN = 2.76 TeV Lint = 0.14 nb-1 0.5 0.5 0 Purity Purity 1 20-40%, |η|<1.37 0 Purity 1 Purity 1 Purity Purity Purity Double sideband method: purity 30 4050 102 2×102 photon p T [GeV] 30 4050 102 2×102 photon p T [GeV] Statistical error assuming A,B,C,D follow multinomial statistics • The purity extracted from the double sideband method is defined as fraction of the tight isolated candidates left after removing backgrounds ! • Beyond dotted lines, statistics in sideband D are small; data extrapolated to sideband regions, and statistical errors reflect fit errors 11 1 0.8 1 Efficiency 1 Efficiency 0.8 0.6 0.6 0.6 0.4 0.4 0.4 0.4 30 40 102 20-40%, |η|<1.37 01 2×102 30 40 2 10 0.2 10-20%, |η|<1.37 01 2 2×10 30 40 2 10 0.2 Efficiency 01 0.2 Efficiency 40-80%, |η|<1.37 photon p T [GeV] 0.8 photon p T [GeV] 0.8 photon p T [GeV] 0.6 0.6 0.6 0.6 0.4 0.4 0.4 0.4 0 30 40 102 0 2×102 photon p T [GeV] 20-40%, 1.52<|η|<2.37 30 4050 102 0.2 0 2×102 photon p T [GeV] 0.8 10-20%, 1.52<|η|<2.37 30 4050 102 0-10%, |η|<1.37 01 2 2×10 0.8 0.2 40-80%, 1.52<|η|<2.37 0.2 ATLAS Pb+Pb Simulation Preliminary 0.8 0.8 0.6 0.2 Efficiency Efficiency 1 Efficiency Efficiency Efficiency in centrality & eta bins 0.2 0 2×102 photon p T [GeV] 30 40 102 2×102 photon p T [GeV] 0-10%, 1.52<|η|<2.37 30 4050 102 2×102 photon p T [GeV] Relative to PYTHIA photons with ETiso<6 GeV at parton level. Includes contributions from reconstruction (η dependence), identification (pT dependence), and isolation (centrality dependence) No systematic uncertainties shown, since effect of cut variations affect both efficiency and purity. Net effect represented in systematics on yield. 12 Systematic uncertainties • Variations applied to various parts of the analysis simultaneously in data and MC, to probe data/MC differences • Photon selection cuts (shower shape, isolation) • Effect of leakage correction • Shower shape corrections removed • Energy scale & resolution uncertainties • Removed fragmentation photons from MC • Evaluated in two regions 22-44.1 GeV, 44.1-111.1 GeV • Largest of each variation selected and applied symmetrically • Total uncertainties vary from 10% to 38%, with larger values in most peripheral events (lower data statistics), forward region (lower statistics), and central events (larger UE fluctuations). 13 n yields per-event yield val inCorrected pT , ⌘ and centrality (C), the per-event yield of leading photons is define sig NA U(pT , ⌘, C) 1 dN (pT , ⌘, C) = Nevt (C) d pT Nevt (C)✏tot (pT , ⌘, C) pT 1. Signal events from sideband analysis 12 2. Energy scale/resolution corrections U (10% from 22-28.1 GeV, then less than 4% for full pT and η range) 3. Nevt from full counting of minimum bias events 4. Efficiency from simulations 5. ∆pT bin width (no scaling by ∆η or ∆ɸ) Contamination from electrons from W estimated to be only appreciable in 2 bins around 40 GeV: 3.5% correction in central η, 5% at forward η 14 7 Photon yields Corrected per-event yield vs. JETPHOX For each interval in p , ⌘ and centrality (C), the per-event yield of leading photons is defined as T sig 10 Central η 106 ATLAS Preliminary Pb+Pb sNN=2.76 TeV |η|<1.37 JETPHOX 1.3 ET,iso(R=0.3) < 6 GeV 12 (dNγ /dp T)/T AA [pb/GeV] 7 107 Forward η 106 3 Data 0-10% × 10 Data 10-20% × 102 Data 20-40% × 101 0 Data 40-80% × 10 T (1/N )(dNγ /dp )/T AA [pb/GeV] NA U(pT , ⌘, C) 1 dN (pT , ⌘, C) = Nevt (C) d pT Nevt (C)✏tot (pT , ⌘, C) pT ATLAS Preliminary Pb+Pb sNN=2.76 TeV 1.52<|η|<2.37 JETPHOX 1.3 ET,iso(R=0.3) < 6 GeV 3 Data 0-10% × 10 Data 10-20% × 102 Data 20-40% × 101 0 Data 40-80% × 10 104 evt 104 (4) 102 102 1 1 10-2 10-2 30 40 50 102 2×102 30 photon p T [GeV] 40 50 60 70 102 photon p T [GeV] 15 102 1 0.5 20-40%, |η|<1.37 2 0 Ratio to JETPHOX (pp ) 0.5 Ratio to JETPHOX (pp ) Ratio to JETPHOX (pp ) 1.5 1 40-80%, |η|<1.37 2 2 10 2 1.5 2 0 2 10 0.5 0-10%, |η|<1.37 2 0 photon p [GeV] photon p [GeV] photon p [GeV] 1.5 1.5 1.5 T 1.5 1 0.5 T 1 0.5 40-80%, 1.52<|η|<2.37 1 0.5 20-40%, 1.52<|η|<2.37 0 2 10 0 2 T T photon p [GeV] T T 0-10%, 1.52<|η|<2.37 10 photon p [GeV] photon p [GeV] 0.5 0 2 102 1 10-20%, 1.52<|η|<2.37 10 photon p [GeV] T ATLAS Preliminary Pb+Pb s NN=2.76 TeV Lint = 0.14 nb-1 1 10-20%, |η|<1.37 Ratio to JETPHOX (pp ) 0.5 0 JETPHOX PDF+scale err. JETPHOX Pb+Pb/ pp JETPHOX EPS09/ pp & err. 1.5 1 2 0 Ratio to JETPHOX (pp ) 1.5 2 Ratio to JETPHOX (pp ) 2 Ratio to JETPHOX (pp ) Ratio to JETPHOX (pp ) Ratios to JETPHOX 102 photon p [GeV] T Ratios to JETPHOX NLO pQCD calculations (CTEQ6.6, BFG II) run with R=0.3/6 GeV isolation. Three configurations: pp (unity), Pb+Pb/pp (black line), EPS09/pp (blue area). Yellow shaded region is scale & PDF uncertainties, shared with Pb+Pb. EPS09 errors represented by blue area. 16 0.8 JETPHOX pp JETPHOX Pb+Pb 0.8 JETPHOX EPS09 0.6 20-40% Lint = 0.14 nb 0.4 0.2 2 2 10 T 0.6 0.2 0 2 10 photon p [GeV] T 0-10% 0.4 0.2 0 photon p [GeV] 0.6 0.4 0.2 0 0.8 10-20% Pb+Pb s NN=2.76 TeV -1 1 Forward: 1.52<|η|<2.37 0.8 Central: |η|<1.37 ATLAS Preliminary 0.6 0.4 1 Forward/Central 40-80% 1 Forward/Central 1 Forward/Central Forward/Central Forward/Central ratios 0 102 10 photon p [GeV] T photon p [GeV] Ratios of forward and central yields, compared with pp (yellow), Isospin (black line), and nPDF EPS09 (blue area). Total uncertainties are large enough to preclude strong statements regarding isospin & nPDF effects. 17 T Conclusions • Centrality, pseudorapidity and pT dependence of photon production in Pb+Pb collisions • First HI measurements at high pT forward η region • Fully corrected yields • Background subtracted, efficiency and resolution corrections • Yields in good agreement with JETPHOX pp • Comparisons made to Pb+Pb with and without nPDF effects • Errors still too large to distinguish among scenarios • Forward/central ratios presented for first time • Sensitive to isospin and nuclear effects • Data systematically lower than pp, but the predictions are close enough that no strong conclusions can be made 18 Extra slides 19 ATLAS dσ / dET [pb GeV-1] JETPHOX CTEQ 6.6 iso ET (∆ R<0.4) < 4 GeV 102 Data 2010 10-1 (a) |η|<0.6 JETPHOX CTEQ 6.6 iso ET (∆ R<0.4) < 4 GeV ATLAS (b) data/theory 10-2 1.4 1.2 1 0.8 0.6 100 150 200 250 300 102 Data 2010 350 400 ET [GeV] ∫ Ldt = 35 pb -1 luminosity uncertainty 10 JETPHOX CTEQ 6.6 iso ET (∆ R<0.4) < 4 GeV ATLAS 1 1.4 1.2 1 0.8 0.6 50 dσ / dET [pb GeV-1] 50 100 150 250 300 Data 2010 10-1 350 400 ET [GeV] ∫ Ldt = 35 pb -1 luminosity uncertainty 10 JETPHOX CTEQ 6.6 iso ET (∆ R<0.4) < 4 GeV ATLAS 1 (c) 200 102 1.52≤|η|<1.81 (d) 1.81≤|η|<2.37 10-2 10-2 1.4 1.2 1 0.8 0.6 50 -1 0.6≤|η|<1.37 10-2 10-1 ∫ Ldt = 35 pb luminosity uncertainty 10 1 data/theory JETPHOX provides a reliable reference over the full phase space in this measurement ∫ Ldt = 35 pb -1 luminosity uncertainty 10 10-1 data/theory 35 of 7 TEV data show quite good agreement with JETPHOX (with a tighter isolation) Data 2010 1 dσ / dET [pb GeV-1] pb-1 102 data/theory JETPHOX vs. ATLAS data dσ / dET [pb GeV-1] Phys.Lett.B 706 (2011) 150-167 100 150 200 250 300 350 400 ET [GeV] 1.4 1.2 1 0.8 0.6 50 100 150 200 250 300 350 400 ET [GeV] Figure 2: Measured (dots) and expected (shaded area) inclusive prompt photon production cross-sections, and their ratio, as a function of the photon ET and in the range (a) |⌘| < 0.6, (b) 0.6 |⌘| < 1.37, (c) 1.52 |⌘| < 1.81 and (d) 1.81 |⌘| < 2.37. The data error bars combine the statistical and systematic uncertainties, with the luminosity uncertainty shown separately (dotted bands). 20
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